The present invention relates to printers and in particular to digital inkjet printers.
Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention on May 24, 2000:
Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending application, PCT/AU00/01445, filed by the applicant or assignee of the present invention on Nov. 27, 2000. The disclosures of these co-pending applications are incorporated herein by cross-reference. Also incorporated by cross-reference, are the disclosures of two co-filed PCT applications, PCT/AU01/00261 and PCT/AU01/00260 (deriving priority from Australian Provisional Patent Application Nos. PQ6110 and PQ6111). Further incorporated is the disclosure of two co-pending PCT applications filed Mar. 6, 2001, application numbers PCT/AU01/00238 and PCT/AU01/00239, which derive their priority from Australian Provisional Patent Application nos. PQ6059 and PQ6058.
Recently, inkjet printers have been developed which use printheads manufactured by micro-electro mechanical systems (MEMS) techniques. Such printheads have arrays of microscopic ink ejector nozzles formed in a silicon chip using MEMS manufacturing techniques. The invention will be described with particular reference to silicon printhead chips for digital inkjet printers wherein the nozzles, chambers and actuators of the chip are formed using MEMS techniques. However, it will be appreciated that this is in no way restrictive and the invention may also be used in many other applications.
Silicon printhead chips are well suited for use in pagewidth printers having stationary printheads. These printhead chips extend the width of a page instead of traversing back and forth across the page, thereby increasing printing speeds. The probability of a production defect in an eight inch long chip is much higher than a one inch chip. The high defect rate translates into relatively high production and operating costs.
To reduce the production and operating costs of pagewidth printers, the printhead may be made up of a series of separate printhead modules mounted adjacent one another, each module having its own printhead chip. To ensure that there are no gaps or overlaps in the printing produced by adjacent printhead modules it is necessary to accurately align the modules after they have been mounted to a support beam. Once aligned, the printing from each module precisely abuts the printing from adjacent modules.
Unfortunately, the alignment of the printhead modules at ambient temperature will change when the support beam expands as it heats up to the operating temperature of the printer.
According to a first aspect, the present invention provides a printhead assembly for an inkjet printer, the printhead assembly including:
a composite support member and a printhead;
said composite support member having a unitary mounting element and an outer shell;
said mounting element and outer shell formed from different materials;
said printhead adapted for mounting to the mounting element and said outer shell adapted for attachment to a printer;
the materials of the support member are selected and structurally combined such that the coefficient of thermal expansion of the support member is substantially equal to the coefficient of thermal expansion of the printhead;
wherein the support member allows limited relative movement between the mounting element and the outer shell.
For the purposes of this specification, xe2x80x9cthe coefficient of thermal expansion of the support memberxe2x80x9d is the effective coefficient of thermal expansion of the mounting element taking into account any external influences from the rest of the support.
Preferably, the printhead is two or more printhead modules that separately mount to the mounting element, each of the modules having a silicon MEMS chip, wherein the mounting element is also formed from silicon.
In a particularly preferred form, the support member further includes a metal portion adapted for attachment to the printer.
Preferably the mounting element is supported by, and adjustably positionable within, the metal portion.
In some embodiments, the printer is a pagewidth printer and the support member is a beam with an elongate metal shell enclosing a central core formed from silicon. Conveniently, the beam is adapted to allow limited relative movement between the silicon core and the metal shell. To achieve this the beam may include an elastomeric layer interposed between the silicon core and the metal shell. Furthermore, the outer shell may be formed from laminated layers of at least two different metals.
It will be appreciated that through careful design and material selection, the coefficient of thermal expansion of the mounting section of the support member can be made to substantially match the coefficient of thermal expansion of the printhead chips. Without any significant differences between the thermal expansion of the printhead and the mounting section of the support member, the problems of printhead module misalignment are avoided. By designing the support member to accommodate some relative movement between the outer shell and mounting section, the problems of bowing are also avoided.